Isolation bearings of the type used with bridges, buildings, machines, and other structures potentially subject to seismic phenomena are typically configured to support a bearing load, i.e., the weight of the structure being supported. In this regard, it is desirable that a particular seismic isolation bearing be configured to support a prescribed maximum vertical gravity loading at every lateral displacement position.
The conservative character of a seismic isolation bearing may be described in terms of the bearing's ability to restore displacement caused by seismic activity or other external applied forces. In this regard, a rubber bearing body, leaf spring, coil spring, or the like may be employed to urge the bearing back to its original, nominal position following a lateral displacement caused by an externally applied force. In this context, the bearing “conserves” lateral vector forces by storing a substantial portion of the applied energy in its spring, rubber volume, or the like, and releases this applied energy upon cessation of the externally applied force to pull or otherwise urge the bearing back to its nominal design position.
Known isolation bearings include a laminated rubber bearing body, reinforced with steel plates. More particularly, thin steel plates are interposed between relatively thick rubber plates, to produce an alternating steel/rubber laminated bearing body. The use of a thin steel plate between each rubber plate in the stack helps prevent the rubber from bulging outwardly at its perimeter in response to applied vertical bearing stresses. This arrangement permits the bearing body to support vertical forces much greater than would otherwise be supportable by an equal volume of rubber without the use of steel plates.
Steel coil springs combined with snubbers (i.e., shock absorbers) are often used in the context of machines to vertically support the weight of the machine. Coil springs are generally preferable to steel/rubber laminates in applications where the structure to be supported (e.g., machine) may undergo an upward vertical force, which might otherwise tend to separate the steel/rubber laminate.
Rubber bearings are typically constructed of high damping rubber, or are otherwise supplemented with lead or steel yielders useful in dissipating applied energy. Presently known metallic yielders, however, are disadvantageous in that they inhibit or even prevent effective vertical isolation, particularly in assemblies wherein the metallic yielder is connected to both the upper bearing plate and the oppositely disposed lower bearing plate within which the rubber bearing body is sandwiched.
Presently known seismic isolation bearings are further disadvantageous inasmuch as it is difficult to separate the viscous and hysteretic damping characteristics of a high damping rubber bearing; a seismic isolation bearing is thus needed which effectively decouples the viscous and hysteretic functions of the bearing.
Steel spring mounts of the type typically used in conjunction with machines are unable to provide energy dissipation, with the effect that such steel spring mounts generally result in wide bearing movements. Such wide bearing movements may be compensated for through the use of snubbers or shock absorbers. However, in use, the snubber may impart to a machine an acceleration on the order of or even greater than the acceleration applied to the machine due to seismicity.
For very high vertical loads, sliding type seismic isolators are often employed. However, it is difficult to control or maintain the friction coefficient associated with such isolators; furthermore, such isolators typically do not provide vertical isolation, and are poorly suited for use in applications wherein an uplift capacity is desired.
One example of an isolation bearing is one used to attempt to reduce the effects of noise by using a rolling bearing between rigid plates. For example, one such device includes a bearing comprising a lower plate having a conical shaped cavity and an upper plate having a similar cavity with a rigid ball-shaped bearing placed therebetween. The lower plate presumably rests on the ground or base surface to which the structure to be supported would normally rest, while that structure rests on the top surface of the upper plate. Thus, when external vibrations occur, the lower plate is intended to move relative to the upper plate via the rolling of the ball-shaped bearing within/between the upper and lower plates. The structure supported is thus isolated from the external vibrations.
However, such devices are not without their own drawbacks. For example, depending on their size, they may have a limited range of mobility. That is, the amount of displacement between the upper and lower plates may be limited based on the size of the bearing. Additionally, the bearing structures may be unstable by themselves. For example, when a large structure is placed on a relatively small bearing, it may become more likely that the structure could tip and/or fall over. Obviously, with very large, heavy structures, such failure could be catastrophic.
Similar to instability, the amount of load that any particular bearing structure can withstand can be limited by its size. Likewise, also related to the instability of the bearing, should the weight of the structure being supported be unevenly distributed, one section of either of the upper or lower plates may tend to bend or deflect more than another and the entire bearing structure could come apart.
Further still, often, when such large structures such as servers, electron microscopes, or other sensitive equipment are to be installed, the buildings and areas into which they are going to be installed are not easily configured to accommodate bearings such as those described above.
Thus, there is a long felt need for vibration isolation structures which can withstand more load, which are more stable (i.e., having less tendency to come apart) and are more easily integrated into the areas into which the structures for which they are intended are to be installed.